1. Field of the Invention
The present invention relates generally to read sensors of magnetic heads in data storage devices, and more particularly to “self-pinned” sensors of the current-perpendicular-to-the-planes (CP) type or the tunnel valve type which have an antiparallel (GAP) self-pinned structure with an GAP self-pinned layer formed in both the central active sensor region and the side regions outside the central region to increase its thermal stability and reduce the likelihood of amplitude flip.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads which include read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magneto resistive (MR) Read sensors, commonly referred to as MR Heads, may be used to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR Sensor detects a magnetic field through the change in the resistance of its MR Sensing layer (also referred to as an “MR Element”) as a function of the strength and direction of the magnetic flux being sensed by the MR Layer. The conventional MR Sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which the MR Element resistance varies as the square of the cosine of the angle between the magnetization of the MR Element and the direction of sense current flow through the MR Element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR Element and a corresponding change in the sensed current or voltage. Within the general category of MR Sensors is the giant magneto resistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR Sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g. Nickel-iron, cobalt-iron, or nickel-iron-cobalt) separated by a layer of nonmagnetic material (e.g. Copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
One of the ferromagnetic (FM) layers referred to as the pinned layer has its magnetization typically pinned by exchange coupling with an antiferromagnetic (AFM) layer (e.g., nickel-oxide, iron-manganese, or platinum-manganese). The pinning field generated by the AFM pinning layer should be greater than demagnetizing fields to ensure that the magnetization direction of the pinned layer remains fixed during application of external fields (e.g. fields from bits recorded on the disk). The magnetization of the other FM layer referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the information recorded on the magnetic medium (the signal field). The pinned layer may be part of an antiparallel (AP) pinned structure which includes an antiparallel coupling (APC) layer formed between first and second AP pinned layers. The first AP pinned layer, for example, may be the layer that is exchange coupled to and pinned by the AFM pinning layer. By strong antiparallel coupling between the first and second AP pinned layers, the magnetic moment of the second AP pinned layer is made antiparallel to the magnetic moment of the first AP pinned layer.
Sensors are classified as a bottom sensor or a top sensor depending upon whether the pinned layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Sensors are further classified as simple pinned or AP pinned depending upon whether the pinned structure is one or more FM layers with a unidirectional magnetic moment or a pair of AP pinned layers separated by the APC layer with magnetic moments of the AP pinned layers being antiparallel. Sensors are still further classified as single or dual wherein a single sensor employs only one pinned layer and a dual sensor employs two pinned layers with the free layer structure located therebetween.
A read sensor may also be of a current-perpendicular-to-the-planes (CPP) type in which current flows perpendicular to the major planes of the sensor layers. First and second shield layers engage the bottom and the top, respectively, of the sensor so as to simultaneously serve as electrically conductive leads for the sensor. The CPP sensor may be contrasted with a current-in-parallel-to-the-planes (CIP) type sensor in which the current is conducted in planes parallel to the major thin film planes of the sensor. In a CPP sensor, when the spacer layer between the free layer and the AP pinned structure is nonmagnetic and electrically conductive (such as copper), the current is referred to as a “sense current”; however when the spacer layer is nonmagnetic and electrically nonconductive (such as aluminum oxide), the current is referred to as a “tunneling current”. Hereinafter, the current is referred to as a perpendicular current Ip which can be either a sense current or a tunneling current.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the sensor to the perpendicular current Ip is at a minimum, and when their magnetic moments are antiparallel the resistance of the sensor to the perpendicular current Ip is at a maximum. A change in resistance of the sensor is a function of cosine θ, where θ is the angle between the magnetic moments of the pinned and free layers. When the perpendicular current Ip is conducted through the sensor, resistance changes, due to field signals from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the sensor is quantified with a magneto resistive coefficient Δr/R, where Δr is the change in resistance of the sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the sensor at minimum resistance.
It should be understood that a narrow track width is important for promoting the track width density of the read head. The more narrow the track width the greater the number of tracks that can be read per linear inch along a radius of the rotating magnetic disk. This enables an increase in the magnetic storage capacity of the disk drive. It should also be understood that the thinner the read gap length, the higher the linear read bit density of the read head. The read gap is the length of the sensor between the first and second shield layers. A relatively thin read gap length means that more bits can be read per inch along the track of a rotating magnetic disk which enables an increase in the storage capacity of the magnetic disk drive.
Assuming that the aforementioned AFM pinning layer is platinum manganese (PtMn), the pinning layer has a thickness of approximately 150 Å. This thickness adversely impacts the linear read bit density of the read head. Further, the pinning layer significantly increases the resistance R of the sensor to the perpendicular current Ip. The result is that the magneto resistive coefficient Δr/R of the sensor is decreased. The pinning layer also requires extra steps in their fabrication and a setting process. It is also important that the free layer be longitudinally biased parallel to the ABS and parallel to the major planes of the thin film layers of the sensor in order to magnetically stabilize the free layer. This is typically accomplished by first and second hard bias magnetic layers which abut first and second sides of the sensor. Unfortunately, the magnetic field through the free layer between the first and second sides is not uniform since a portion of the magnetization is lost in a central region of the free layer to the shield layers. This is especially troublesome when the track width of the sensor is of sub-micron dimensions. End portions of the free layer which abut the hard bias layers are over-biased and become very stiff in their response to field signals from the rotating magnetic disk. The stiffened end portions can take up a large portion of the total length of a sub-micron sensor and can significantly reduce the amplitude of the sensor.
Instead of having an AFM pinning layer which pins the AP pinned structure, the read sensor may alternatively have an AP “self-pinned” structure. A read sensor of the self-pinned type relies on magnetostriction of the AP self-pinned structure and the ABS stress for a self-pinning effect. The AFM pinning layer, which is typically as thick as 150 Angstroms, is no longer necessary for pinning purposes so that a relatively thin sensor can be advantageously fabricated. A self-pinned structure can achieve higher bit densities with its thinner profile and increased sensitivity.
For self-pinned sensors, it has been necessary to improve the magnetic pinning field in order to prevent amplitude flipping. During disk drive operation, readback signals from the disk are detected as either a “0” or “1” depending on the polarity of the bits recorded on the disk. When an undesirable head-to-disk interaction occurs (e.g. from defects, asperities, bumps, etc.), the sensor experiences compressive or tensile stress which may cause the pinned layers to flip orientation. Electrical overstress (EOS) from electrostatic discharge (ESD) in the sensor during manufacturing and/or handling may also induce such flipping. If the sensor is of the CPP type, current flows through the sensor in a direction that is perpendicular to the layers which increases the sensor's temperature, to thereby produce a source of thermal stress which further contributes to the likelihood of amplitude flip. Due to these sources of stress, the pinned layers may flip their direction either permanently or semi-permanently depending on the severity of the stress. This causes the amplitude of the readback signal to flip (hence the terminology “amplitude flip”), which results in corrupt data.
Accordingly, there is an existing need to overcome these and other deficiencies of the prior art.